As the demands on the versatile wireless communication service, such as voice, data and multimedia, increase rapidly, the wireless local area network (WLAN) also grows fast. To meet the ever-growing demands, more and more wireless nodes are used in addition to the wired service. As the technology matures and the cost reduced, WLAN is gaining popularity.
In a WLAN, an access point (AP) is usually individually set up without the overall planning. Under such circumstances, if a nearby AP uses the same frequency, the mutual interference will exist between the APs. Therefore, it is imperative to prevent such interference and maintain a high carrier-to-interference ration (CIR) to guarantee the high data rate.
In recent years, a multiple-input-multiple-output (MIMO) multi-user technology is developed. This technology greatly reduces the co-channel interference (CCI) and increases the system capacity. In MIMO technology, every AP and every subscriber use a plurality of antennas to serve the multiple users in the same frequency band. This technology is considered as a viable solution for the next generation mobile communication and WLAN. In the MIMO technology, the subscribers at different angular positions can share the same network resource if the separation angle among the subscribers is sufficiently large.
After MIMO is applied to a WLAN, the channel assignment plays an important role in increasing the network throughput. In the conventional wireless communication network, channel assignment is categorized, based on the CCI elimination strategy, into three categories. The first is the fixed channel assignment (FCA), in which a cell is assigned a pre-determined set of channels. The FCA method is simple, yet suffers poor performance when the traffic is non-stationary or non-uniform.
The second is called dynamic channel assignment (DCA). The conventional DCA allows all the channels in the universal set to be assigned to any service requesting subscriber as long as the CIR is within the limit. The conventional DCA has the problems of higher complexity and requiring the feedback of a large amount of channel state information.
The third category is the hybrid channel assignment (HCA). This method is suitable for compromise between the complexity and the performance. The conventional DCA and the HCA both have good performance for mobile systems with the same cells. Unfortunately, to provide service to the subscribers around the hot spot, the placement of APs in the WLAN is always non-uniformly distributed. Even worse, it is not easy to measure the CIR at the receiving end, and higher CIR does not account for good service quality before the antenna beam forming and frequency equalization.